What Is a Stink Worm? Identifying the Ailanthus Webworm

The term “stink worm” is a common nickname for the caterpillar of the ailanthus webworm moth, Atteva aurea. This name can be misleading, as the creature is not a true worm but the larval stage of a moth that belongs to a group known as ermine moths. Originally native to tropical regions of Central and South America, this insect has expanded its range northward with its primary food source. The ailanthus webworm is now a common sight in many parts of the United States.

Identifying the Ailanthus Webworm

The caterpillar, or larva, is slender and can grow up to an inch long. Its body is brownish to nearly black, marked with a prominent light greenish-brown stripe down its back. Along its sides are several thin, alternating stripes of white and olive-green. These caterpillars are also covered in sparse, short hairs that help suspend them within their webs.

A primary sign of an infestation is messy, communal webs spun from silk. The caterpillars create these structures by pulling together leaflets of their host plant, forming a protective nest where they live and feed in groups. The adult moth is equally distinct, with a slender body about half an inch long. Its bright orange wings are covered with white spots ringed in black, and when at rest, it holds its wings tightly against its body, resembling a beetle more than a moth.

The “Stink” Defense Mechanism

The “stink worm” moniker comes from a defense mechanism, though the caterpillar itself does not produce an odor. The bad taste and smell are acquired directly from its diet. The webworm’s primary food source in North America is the Tree of Heaven (Ailanthus altissima), a plant known for producing bitter chemical compounds in its leaves, bark, and roots.

As the caterpillars feed on the Tree of Heaven, they ingest and store these chemicals within their bodies. This process makes the caterpillars unpalatable to potential predators like birds and other insects. The bright coloration of both the caterpillar and the adult moth serves as a warning signal to predators, advertising their foul taste. This form of chemical defense allows the webworm to survive and reproduce successfully.

Ecological Role and Human Interaction

The ailanthus webworm plays an ecological role through its relationship with the Tree of Heaven. This tree is an invasive species from China that grows aggressively, outcompeting native plants and disrupting local ecosystems. The webworm feeds almost exclusively on this invasive tree, acting as a form of natural biological control. By consuming the leaves, the caterpillars can help manage the tree’s spread, although they rarely cause enough damage to kill large, established trees.

For homeowners and gardeners, the ailanthus webworm is generally not a cause for concern. The caterpillars are harmless to humans and pets; they do not possess a sting or a bite. Since their diet is highly specialized to the invasive Tree of Heaven and a few related tropical plants, they pose no threat to garden ornamentals or agricultural crops. Because they target an invasive plant, control measures are seldom necessary or recommended.